According to the RHS website, the female lays eggs into the leaf stems. The larve then feed on the leaves, making small holes first, then eventually stripping the leaves of foliage (mine have got to this stage !). Once the larvae have got full tummies, they then overwinter in the soil and re-emerge the following spring.

The organic method to rid the plants of these pests is to pick them off (eeeek and double yuck !!).

The chemical method is to spray the plants. Chemicals recommended are Bayer Provado Ultimate Bug Killer,
Bayer Sprayday Greenfly Killer or Py Spray Garden
Insect Killer, Doff All in One Bug Spray. Preferable to do this whilst the larvae are still small - these ones definitely aren't small, but I've got to try something otherwise my plant isn't going to be big and strong next year. I'll miss its lovely display of white flowers in the spring :-(

We have had these in our garden for many years. As they don't appear until the blooming period of the plant is complete, we just let it happen.The great variety of insects in gardens can only be sustained if we tolerate those who are not downright harmful.